The ubiquitin-proteasome system is crucial in controlling the abundance of key regulators of a myriad of cellular processes. The N-end rule and the ubiquitin fusion degradation (Ufd) pathways are two ubiquitin-proteasome degradation pathways genetically defined in the budding yeast and evolutionarily conserved. Two major classes of ubiquitin protein ligases (E3s) responsible for substrate selection in the ubiquitin-proteasome system are represented in these pathways: the RING domain ligase and the HECT domain ligase. The cellular processes regulated by these degradation pathways are presently unclear because few cellular substrates have been identified. The goals of the proposed research are: 1) To identify cellular substrates degraded by the ubiquitin-proteasome pathway by measuring changes in protein abundance upon proteasome inhibition. 2) To identify those proteasome targets degraded via the N-end rule and Ufd pathways by analyzing changes in protein abundance in the E3 deleted cells. 3) To use this information to unveil the cellular processes controlled by these degradation pathways. The regulatory switches of the N-end rule and Ufd degradation pathways will be characterized by using a combination of detailed biochemical and genetic techniques.